The present invention relates to solvent extraction processes for the recovery of metal values from aqueous solutions.
In the process for extracting copper from a copper pregnant aqueous acidic leach solution (PLS) obtained by the extraction of copper from copper ores, wherein a water-immiscible organic solvent solution containing an extraction reagent is mixed with the copper pregnant aqueous acidic leach solution to form a copper loaded organic solvent solution, trace amounts of Fe(III) as well as other impurities such as Mn(II) from the acidic leach solution are also transferred to the organic solvent solution by chemical loading as well as by entrainment of the acidic leach solution. When an aqueous electrolyte solution is used to strip the copper from the loaded organic solvent solution, the Fe(III) and other impurities contained in the entrained acidic leach solution present in the organic solvent solution and the Fe(III) and other impurities chemically loaded therein are transferred to the aqueous electrolyte strip solution.
Hence, a variety of impurities can thereby be transferred to the electrowinning tankhouse from the aqueous electrolyte strip solution. These impurities can cause difficulties in the electroplating of copper. Manganese(II) and chloride ions are examples of impurities that reach the tankhouse by entrainment. Manganese(II) can be converted to insoluble manganese dioxide at the anode surface, where it precipitates. The manganese dioxide layer causes some spalling of the anode surface to form fine lead particles that in turn become incorporated into the copper cathode, resulting in poor quality cathode. Chloride can be converted to chlorine gas at the anode representing a health hazard for the tankhouse personnel. The presence of high levels of chloride in the tankhouse also results in pitting of the stainless steel blanks used to plate the copper which in turn results in cathode sticking, requiring labor intensive (expensive) manual stripping of the copper cathodes. As discussed above, iron(III) is an example of an impurity that is transferred by chemical loading. The presence of high levels of iron in the electrolyte results in high plating costs due to poor current efficiency. Iron(III) is reduced to iron(II) at the cathode while iron(II) is converted to iron(III) at the anode effectively causing a short circuit in the electrowinning cell. While a small amount of Fe is desirable in the tankhouse (0.5-1.5 gpl), higher levels are undesirable due to the effect on current efficiency. To control the Fe in the tankhouse electrolyte, operators periodically bleed the tankhouse and replace the bled volume with fresh water, sulfuric acid, and electrowinning additives such as guar, cobalt sulfate, and anti-misting aids. This results in substantial cost. The bleed is typically mixed with incoming PLS and fed to extraction to recover the copper, or a portion is added to the wash stage to control the acidity in the wash stage aqueous and provide some additional copper which is extracted by the reagent and helps to crowd the Fe(III) off the organic. While the acid values in the bleed are not truly lost since they eventually find their way to the acid leaching solution used to leach copper from copper ore, the other tankhouse additives in the bleed are lost and represent a substantial cost.
In order to minimize the transfer of Fe(III) from the loaded organic solvent solution to the aqueous electrolyte strip solution, one or more wash stages using at least a portion of electrolyte as the wash solution were introduced into the process to wash the loaded organic solvent solution prior to its contact with the aqueous electrolyte strip solution. This invention relating to the use of at least a portion of electrolyte as the wash solution forms the subject matter of U.S. Pat. No. 4,957,714.
It has now been discovered that when the aqueous phase used in the wash stage is first contacted with copper metal, a significant increase in the removal of iron ions from the loaded organic solvent solution is obtained when the wash aqueous phase is contacted therewith, i.e. iron scrubbing efficiency is significantly improved.
It has also been discovered that increasing the temperature of the wash aqueous phase before contact with the copper metal further enhances the iron scrubbing efficiency of the aqueous phase.